Past research on immigrant health frequently finds that the duration of time lived in the United States is associated with the erosion of immigrants’ health advantages. However, the timing of U.S. migration during the life course is rarely explored. We draw from developmental and sociological perspectives to theorize how migration during childhood may be related to healthy eating among adult immigrants from Mexico. We test these ideas with a mechanism-based age-period-cohort model to disentangle age, age at arrival, and duration of residence. Results show that immigrants who arrived during preschool ages (2–5 years) and school ages (6–11 years) have less healthy diets than adult arrivals (25+ years). After accounting for age at arrival, duration of residence is positively related to healthy eating. Overall, the findings highlight the need to focus more research and policy interventions on child immigrants, who may be particularly susceptible to adopting unhealthy American behaviors during sensitive periods of childhood.